Donald Savage
NASA Headquarters, Washington, DC November 3, 1999
(Phone: 202/358-1547)
Amber Jones
National Science Foundation, Arlington, VA
(Phone: 703/306-1070)
RELEASE: 99-127
ASTRONOMERS FIND EVIDENCE OF FIRST PLANET ORBITING A PAIR OF STARS
Astronomers have found evidence of the first known planet
orbiting a pair of stars. Previously, planets have been found
circling only single stars.
The Microlensing Planet Search (MPS) project, led by David
Bennett and Sun Hong Rhie of the University of Notre Dame, South
Bend, IN, used a technique called gravitational microlensing that
may have revealed a planet about three times the mass of Jupiter
orbiting a binary star system. The researchers, who are supported
by NASA's Astronomical Search for Origins Program, the National
Science Foundation (NSF) and the Research Corporation, report
their results in the November 4 issue of Nature.
"Between half and two-thirds of the stars in our solar
neighborhood are known to be members of binary or multiple star
systems," said Morris Aizenman of NSF's Astronomical Sciences
Division. "To find evidence of a planet orbiting a pair of stars
means there could be more planetary systems than we previously
thought." Astronomers have detected only about 20 planets outside
our solar system, all orbiting single stars, although some of
those stars are in binary systems.
Gravitational lensing is based on a property first noted by
Albert Einstein in the 1930s. When an object such as a star or
planet moves in front of a more distant star, the gravity of this
star or planet serves as a "lens," magnifying the light from the
distant star and making it appear brighter. The Microlensing
Planet Search astronomers analyzed data from such an event that
occurred in 1997, referred to as MACHO-97-BLG-41 -- the 41st
microlensing event discovered by the Massive Compact Halo Objects
(MACHO) collaboration that year. During this 100-day event, the
pattern of brightness appeared too complex to be produced by a
single-star lens.
While Bennett and his colleagues believe the best model for
explaining this microlensing event is a planet orbiting a binary
star system, other astronomers have proposed alternative models
they believe could also fit the data. One possibility is that the
orbital motion of the binary star system itself could have caused
the change in the observed brightness of the distant star.
Another possibility is that the distant star may itself be part of
a binary system. These scenarios will be tested in future
observations.
The MACHO project, which is supported by NSF as part of the
National Science and Technology Center for Particle Astrophysics
at the University of California at Berkeley, routinely makes data
on microlensing events available to other astronomers. MACHO is
using microlensing to explore tens of millions of stars in a
search for the "dark matter" that dominates the mass of our
galaxy. Dark matter is believed to exist because the combined
gravity of the known matter in the universe is not enough to
account for the observed gravitational effects.
The MPS astronomers are using the microlensing technique to
search for planets orbiting stars other than our Sun. For this
analysis, they used observations from telescopes at the Mount
Stromlo Observatory in Australia and the Wise Observatory in
Israel as well as data from the NSF's Cerro-Tololo Inter-American
Observatory in Chile. Astronomers at the Wise Observatory co-
authored the Nature report.
More information on MPS can be found on the Internet at:
http://bustard.phys.nd.edu/MPS/
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